1. Field of the Invention
The present invention relates to a recording medium made of a photo refractive material, i.e., a so-called holographic memory, and particularly relates to a hologram recording method and an optical information recording and reproducing apparatus utilizing the holographic memory.
2. Description of Related Art
A volume holographic recording system is known as a digital information recording system utilizing the principle of a hologram. This system is characterized in that an information signal is recorded onto a recording medium as a change in refractive index. A photo refractive material such as a lithium niobate monocrystal is used in the recording medium.
Explanation will be made for a recording and reproducing method using a Fourier transformation as an example of conventional hologram recording and reproducing methods.
As shown in FIG. 1, in a conventional 4f-system hologram recording and reproducing apparatus, a laser beam 12 emitted from a laser beam source 11 is divided into a signal light beam 12a and recording reference light beam 12b at a beam splitter 13. The diameter of the beam the signal light beam 12a is enlarged by a beam expander BX, and is irradiated as parallel light to a spatial light modulator SLM such as a panel of a TFT liquid crystal display (LCD) of a transmission type. The spatial light modulator SLM receives recording data converted to a signal by an encoder as an electric signal, and forms a light and dark dot pattern on a plane. When the signal light beam 12a is transmitted through the spatial light modulator SLM, the signal light beam 12a is optically modulated to include a data signal component. The signal light beam 12a including the dot pattern signal component is transmitted through a Fourier transformation lens 16 separated by its focal length f from the spatial light modulator SLM so that the dot pattern signal component is Fourier-transformed and is converged into a recording medium 10. In contrast, the recording reference light beam 12b divided in the beam splitter 13 is guided into the recording medium 10 by mirrors 18, 19, and crosses an optical path of the signal light beam 12a within the recording medium 10 so that a light interference pattern is formed. The entire light interference pattern is recorded as a change in refractive index.
Thus, diffracted light from image data which is irradiated by coherent parallel light is focused and formed as an image by the Fourier transformation lens, and is reformed to a distribution on a focal face of the Fourier transformation lens, i.e., on a Fourier face. The distribution of results of the Fourier transformation interferes with the coherent reference light, and its interference fringe is recorded to the recording medium in the vicinity of a focal point. After the recording operation on a first page is terminated, a rotatable mirror is rotated by a predetermined amount and its position is displaced in parallel by a predetermined amount so that an incident angle of the recording reference light beam 12b with respect to the recording medium 10 is changed. The recording operation on a second page is then performed in the same procedure. Thus, an angle multiplex recording is performed by executing the sequential recording operation.
On the other hand, a dot pattern image is reproduced by performing a reverse Fourier transformation at the time of reproduction. When reproducing information, as shown in FIG. 1, for example, the optical path of the signal light beam 12a is interrupted by the spatial light modulator SLM, and only the recording reference light beam 12b is irradiated to the recording medium 10. At the time of reproduction, a position and an angle of the mirror are changed and controlled in combination with the rotation and the linear movement of the mirror so as to provide the same incident angle as the recording reference light at the recording time of a reproduced page.
Reproduction light as a result of reproduction of the recorded light interference pattern appears on an opposite side of the recording medium 10 irradiated by the recording reference light beam 12b. When this reproduction light is guided to a reverse Fourier transformation lens 16a and the reverse Fourier transformation is performed, the dot pattern signal can be reproduced. Further, after this dot pattern signal is received by a photo detector 20 such as a charge coupled device CCD in a focal length position, the original data is reproduced by reconverting the received dot pattern signal to an electric digital data signal and then sending it to a decoder.
Thus, as shown in FIG. 2, the multiple recording has conventionally been made within the volume of about several millimeters in square using angle multiplex and wavelength multiplex to record information at high density within a certain volume within the recording medium. Therefore, coherent lengths of the signal light and the reference light are set to be long and wide in order to secure properties that allow selecting angle and wavelength broadly. This leads to a reduction in intensity per light unit used in the recording, the recording medium of high sensitivity is required. Furthermore, since the multiplex record is required to make the high density record, a recording medium having a large erasing time constant and able to be easily multiplexed is required. The recording medium having a large erasing time constant is generally low in recording sensitivity, and is a cause of the limited recording speed as a system.
Further, two lenses constituted by the Fourier transformation lens and the reverse Fourier transformation lens of high performance are required in the conventional device. Furthermore, it is necessary to arrange a paging control mechanism of high accuracy to control the reference light in the recording and the reproduction. Accordingly, a problem exists in that it is disadvantageous to make the system compact.
Furthermore, in the case of conventional arrangements the multiplex recording is executed in one portion of the recording medium, changes in refractive index of the recording medium due to the multiplex recording are added and a wave front of the reference light after the medium transmission is gradually changed. Therefore, the number of multiplexing is limited in order to reflect and set the reference light to reproducing reference light and perform the generation of phase conjugation. Furthermore, it has been necessary to arrange a light interrupting means for the reflected light so as to remove the overlapping of influences due to the formation of a diffraction grating in a reducing direction of the recording sensitivity in a multiplex recording time.